Is this sensor dust?

Yeah, it looks like sensor dust to me. Drop it off at your local camera store and have them clean it for you. Or you can buy a cleaning kit and do it yourself if you're comfortable.

4 years is a pretty long cleaning interval. Don't blow canned air in it or use non-approved cleaning agents. You can start out with a blower (the Rocket Blower series tend to be very well received) and see if that clears it before moving on to either taking it somewhere or going to sensor swabs and cleaner. You'll want to blow it out before swabbing anyway so you don't take the risk of scratching the AA filter or hot mirror when you do swab it.

Yup. Definitely sensor dust.

To clean, try bulb blower first, then swabs (if needed), then something like the Arctic Butterfly (if needed). I generally use the blower and a swab every few weeks, and Arctic Butterfly before every session. I don't have a sensor dust problem anymore.

How do you check if you have sensor dust?
i have nikon d60

Best to take a shot on a white background. No flash, wide open.

Actually, you want to stop down fully instead of shooting wide open, this will reveal more and smaller dust particles that would be blurred out wide open. This is because stopping down increases the depth of field both in front of the lens and behind, where the sensor is. A greater depth of field behind the lens will bring more dust in to focus.

Extend your lens out all the way (if it's a zoom) and manual focus on infinty. Stop down fully and shoot a white wall or other featureless scene. I usually move the camera around deliberately during the exposure to blur out any details that may be in the scene (wall texture, etc) and also help even out the illumination across the scene too. Since the dust on the sensor is not going to move, the dust stays sharp but the rest of the image is more uniform.

Ruahrc

A side note, the dust vibration mechanism in Olympus dslr is flawless and has been for years. I never get dust on the sensor, and shoot constantly under harshest conditions even changing lenses on a windy beach frequently.

Not that Oly is perfect in every regard, to the contrary... but you other brand shooters should really get on your manufacturers about making something similar, that does not require a professional service or the risk of damaging a critical piece or camera's functionality. Squeaky wheel gets the grease.

Yep sensor dust. Contrary to what another posted I would say stop down and shoot on a white wall. Wide open you may not see the dust. I would suggest however not stopping down ALL the way or youll be OCD trying to remove everything you see. (To see what I mean attach a pinhole attachment to your "clean sensor" and shoot )